Many petroleum fractions used for the manufacture of fuels and in petrochemicals processes often contain organic sulfur and nitrogen compounds as contaminants. To comply with relevant regulatory standards for fuels, these fractions need to be reduced to lower levels. Reduction of these contaminants is also required when the fractions are to be treated in subsequent refining processes if the presence of these contaminants in the feed leads to poisoning of the catalysts used in the processes. Reforming and isomerization, for example, typically demand no more than 10 ppmw sulfur in the feed and many catalyst manufacturers recommend no more than 1 ppmw with certain types of catalyst.
A common feature of petroleum processing equipment is the surge drum which is a vessel designed to accommodate differences between the rate at which a fraction is received in the unit (or part of it) and the instantaneous rate at which it is to be fed to subsequent processing steps. With hydrocarbon streams, it is the general practice to carry out some form of inerting under mild positive pressure in order to preclude entry of outside air with its consequent risk of explosion. A number of inerting or blanketing gases are available, for example, nitrogen, and in many petroleum refineries natural gas or refinery fuel gas provides a readily available and convenient blanketing gas. Some of these gases have, however, been found to have undesirable effects on processing with certain catalysts, particularly those containing catalytically active metals.
Among the catalysts susceptible to deactivation are those used in the ExxonMobil selective naphtha hydrofining process, SCANfining™, developed for deep hydrodesulfurization of catalytically cracked naphthas with maximum preservation of the olefins (octane). With this process it has been found, as noted in US2012/0241360, that the presence of carbon monoxide (CO), carbon dioxide (CO2) or mixtures of the two may inhibit the action of the catalyst(s). If these gases are present in minor amounts the catalysts will still function satisfactorily but if they are present in excessive quantities, they will inhibit the desulfurization activity of the catalysts. Since the inhibition is less significant on the olefin saturation reaction, the presence of CO and CO2 in the treat gas results in an increased octane loss as a higher degree of olefin saturation will take place as conditions are modified to achieve a constant level of desulfurization resulting a higher olefin saturation which increases the octane loss and decreases product quality. Table 1 below illustrates the effect of carbon monxide on the catalyst normally used in the process.
TABLE 1CO inhibition on SCANfining Catalyst PerformanceCO concentration in treat gasCatalyst Activity Reduction, %30 ppmv45 ppmvDesulfurization Reaction3340Olefin-Saturation Reaction1820
A similar effect can be applied to CO2 since CO and CO2 will be in the equilibrium state governed by the water gas shift reaction. The CO+CO2 concentration in the treat gas should be as low as possible, preferably less 10 ppmv to minimize their inhibition of the catalytic reactions.
While the CO+CO2 composition of the treat gas is generally maintained by keeping the make-up hydrogen purity within tightly controlled limits to ensure proper functioning of the catalysts, the composition of the blanketing gas in the surge drum(s) has not previously been considered to be a significant factor in process design. However, as a result of investigation, it has been shown that the CO and CO2 in the blanketing gas may dissolve in the liquid feed stream and so come into contact with the catalyst to the detriment of catalyst activity. Accordingly, it is necessary to define acceptable levels of these gases in the blanketing gas and provide methods for their control.